Method of fabricating semiconductor device

ABSTRACT

A metal target, at least the surface region of which has been oxidized, is prepared in a chamber. Then, a sputtering process is performed on the metal target with an inert gas ambient created in the chamber, thereby depositing a first metal oxide film as a lower part of a gate insulating film over a semiconductor substrate. Next, a reactive sputtering process is performed on the metal target with a mixed gas ambient, containing the inert gas and an oxygen gas, created in the chamber, thereby depositing a second metal oxide film as a middle or upper part of the gate insulating film over the first metal oxide film.

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor device including a gateinsulating film made of a material with a high dielectric constant(which will be herein referred to as a “high-dielectric-constantmaterial”) and also relates to a method for fabricating the device.

Recently, there has been a growing demand for high-speed-operatingsemiconductor devices. To meet this demand, the gate insulating film ofMOSFETs has been further thinned for the purpose of increasing thedrivability thereof.

However, if the gate insulating film is a thin film of SiO₂ (with arelative dielectric constant ∈ of 3.9), the gate leakage currentincreases noticeably because a tunneling current flows therethrough.

Thus, to prevent the gate leakage current from increasing whileenhancing the drivability of MOSFETs, various methods for increasing theactual thickness of the gate insulating film and obtaining a desiredgate capacitance have been researched. For example, according to one ofthose methods, the gate insulating film is made of ahigh-dielectric-constant material (high-κK material) such as HfO₂(hafnium dioxide with a relative dielectric constant ∈ of about 30) orZrO₂ (zirconium dioxide with a relative dielectric constant ∈ of about25).

To deposit a gate insulating film of a high-dielectric-constantmaterial, a reactive sputtering process is performed using a target ofHf or Zr, for example, in a mixed gas ambient containing Ar (argon) andO₂ gases, for example. In this manner, a gate insulating film of ahigh-dielectric-constant material such as HfO₂ or ZrO₂ can be depositedover a semiconductor substrate.

However, if the gate insulating film of the high-dielectric-constantmaterial is deposited over a silicon substrate by the reactivesputtering method, for example, the surface of the silicon substrate isoxidized by a plasma created from the O₂ gas during the reactivesputtering process. Thus, an unwanted silicon dioxide film is formedbetween the silicon substrate and gate insulating film. It should benoted that the unwanted film will be herein referred to as a “silicondioxide film” but can actually be any other silicon oxide film with anon-stoichimetric composition. Consequently, the gate insulating filmbecomes a stack of the silicon dioxide film with a relatively lowdielectric constant and the high-dielectric-constant film. As a result,the gate insulating film has its effective dielectric constant decreasedas a whole.

That is to say, the known method for fabricating a semiconductor devicecannot obtain the desired gate capacitance. Thus, it is difficult toenhance the drivability of MOSFETs.

FIG. 7 is a cross-sectional view showing the known method forfabricating a semiconductor device.

As shown in FIG. 7, a target 80 of Zr is placed in a chamber (not shown)and a silicon substrate 90 is loaded thereto. Then, a reactivesputtering process is performed using the target 80 with a mixed gasambient containing Ar and O₂ gases created in the chamber. During thisprocess, the surface of the target 80 is oxidized, thereby forming a Zroxide layer 81 thereon. At the same time, the surface of the siliconsubstrate 90 is also oxidized to be covered with a silicon dioxide film91. Further, as a result of the reactive sputtering process, a Zr oxidefilm 92 is formed over the silicon substrate 90 with the silicon dioxidefilm 91 interposed therebetween. Accordingly, the resultant gateinsulating film becomes a stack of the silicon dioxide film 91 and Zroxide film 92. As a result, the gate capacitance decreases compared to agate insulating film that has the same thickness but consistsessentially of a Zr oxide film alone.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to enhance thedrivability of MOSFETs by getting a gate insulating film, consistingessentially of a high-dielectric-constant material alone, formed by asputtering process without allowing any silicon dioxide film to exist onthe surface of a semiconductor substrate.

An inventive method for fabricating a semiconductor device includes thesteps of: a) preparing a metal target in a chamber, at least a surfaceregion of the target having been oxidized; b) performing a sputteringprocess using the metal target with an inert gas ambient created in thechamber, thereby depositing a first metal oxide film as a lower part ofa gate insulating film over a semiconductor substrate; and c) performinga reactive sputtering process on the metal target with a mixed gasambient, containing the inert gas and an oxygen gas, created in thechamber, thereby depositing a second metal oxide film as a middle orupper part of the gate insulating film over the first metal oxide film.

According to the inventive method, in the step of depositing a firstmetal oxide film over a semiconductor substrate, i.e., the initial stageof a process for forming a gate insulating film, no reactive sputteringprocess is performed but a sputtering process is performed using a metaltarget, at least the surface region of which has been oxidized, in anambient containing no oxygen gas. Thus, the first metal oxide film canbe deposited over the semiconductor substrate without allowing anysilicon dioxide film to exist on the surface of the semiconductorsubstrate. Also, in the step of depositing a second metal oxide filmover the first metal oxide film, i.e., after the initial stage of theprocess for forming the gate insulating film is over, a reactivesputtering process is performed in an ambient containing an oxygen gaswith the surface of the semiconductor substrate covered with the firstmetal oxide film. Thus, the second metal oxide film can be depositedover the first metal oxide film without allowing any silicon dioxidefilm to exist on the surface of the semiconductor substrate.Accordingly, the gate insulating film can be essentially made up of thefirst and second metal oxide films alone. In other words, a gateinsulating film consisting essentially of a high-dielectric-constantmaterial alone can be formed. As a result, the resultant MOSFET can haveits gate capacitance increased and its drivability enhanced. Inaddition, a gate leakage current can be minimized because the gateinsulating film can be thick enough with a desired gate capacitancemaintained.

In one embodiment of the present invention, the step a) may include thestep of performing a provisional reactive sputtering process on themetal target to be oxidized with a mixed gas ambient, containing theinert and oxygen gases, created in the chamber, thereby oxidizing thesurface region of the metal target before the semiconductor substrate isloaded into the chamber.

Then, the metal target with the oxidized surface region can be preparedeasily.

In this particular embodiment, the provisional reactive sputteringprocess is preferably performed on another semiconductor substrate thathas been loaded into the chamber before the step a) is started.

Then, no insulating metal oxide is deposited on a wafer stage (whichwill be used as a gas-discharge electrode during the subsequentsputtering process steps) in the chamber when the surface region of themetal target is oxidized. As a result, it is possible to avoid theinability to apply a voltage to the semiconductor substrate in thesubsequent process steps.

In another embodiment, the step c) may include the step of introducingthe oxygen gas into the chamber with the inert gas, used in the step b),left in the chamber and with a gas-discharge continued from the step b)to carry out the reactive sputtering process.

Then, the steps b) and c) of depositing the first and second metal oxidefilms can be performed continuously. As a result, the throughput of theprocess improves.

In an alternative embodiment, the inventive method may further include,between the steps b) and c), the step of introducing the oxygen gas intothe chamber with the inert gas, used in the step b), left in the chamberand with a gas-discharge for the sputtering process suspended.

Then, the mixture ratio of the inert and oxygen gases can be fixedbefore the step c) of depositing the second metal oxide film is started.As a result, the oxygen concentration of the second metal oxide film iscontrollable more easily.

In another alternative embodiment, the inventive method may furtherinclude, between the steps b) and c), the step of exhausting the inertgas, used in step b), from the chamber and then newly introducing theinert gas along with the oxygen gas into the chamber with agas-discharge for the sputtering process suspended.

Then, the mixture ratio of the inert and oxygen gases should be fixedbefore the step c) of depositing the second metal oxide film is started.As a result, the oxygen concentration of the second metal oxide film iscontrollable much more easily.

In yet another embodiment, the step c) may include the step of supplyingthe oxygen gas at a controlled flow rate into the chamber to deposit thesecond metal oxide film with a different oxygen concentration from thatof the first metal oxide film.

Then, the structure of the gate insulating film can be optimized withthe reliability and the dielectric constant of the gate insulating filmboth taken into account. As a result, a highly reliable,high-performance MOSFET is realized.

An inventive semiconductor device includes a gate insulating film thatincludes: a first metal oxide film deposited on a semiconductorsubstrate; and a second metal oxide film deposited on the first metaloxide film. In this device, the first and second metal oxide films aremade of the same type of metal oxide and have mutually different oxygenconcentrations.

In the inventive device, the structure of the gate insulating film hasbeen optimized with the reliability and the dielectric constant of thegate insulating film both taken into account. Thus, the device isimplementable as a highly reliable, high-performance MOSFET.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A through 1D are cross-sectional views showing respective processsteps of a semiconductor device fabricating method according to a firstembodiment of the present invention.

FIG. 2 shows variations in flow rates of Ar and O₂ gases with time and avariation in gas-discharge power for sputtering with time in the methodof the first embodiment.

FIG. 3 is a graph showing the oxygen concentration in the thicknessdirection of Zr oxide films deposited by the method of the firstembodiment.

FIG. 4 is a graph showing the oxygen concentrations in the thicknessdirection of Zr oxide films deposited by a semiconductor devicefabricating method according to a modified example of the firstembodiment.

FIG. 5 shows variations in flow rates of Ar and O₂ gases with time and avariation in gas-discharge power for sputtering with time in asemiconductor device fabricating method according to a second embodimentof the present invention.

FIG. 6 shows variations in flow rates of Ar and O₂ gases with time and avariation in gas-discharge power for sputtering with time in asemiconductor device fabricating method according to a third embodimentof the present invention.

FIG. 7 is a cross-sectional view showing a known method for fabricatinga semiconductor device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiment 1

Hereinafter, a semiconductor device and a method for fabricating thedevice according to a first embodiment of the present invention will bedescribed as being applied to an NMOSFET with reference to the drawings.

FIGS. 1A through 1D are cross-sectional views showing respective processsteps for fabricating a semiconductor deice according to the firstembodiment. In this method, a sputtering system of a single waferprocessing type is used.

First, as shown in FIG. 1A, a target 10 of Zr is placed in a chamber(not shown) and a dummy silicon substrate 20 is loaded thereto. Then, areactive sputtering process is provisionally, performed using the Zrtarget 10 with a mixed gas ambient containing Ar and O₂ gases created inthe chamber. In this manner, a Zr oxide layer 11 having a thickness ofabout 5 nm to about 10 nm is formed in the surface region of the Zrtarget 10. In the meantime, a Zr oxide film 21 is deposited to athickness of about 10 nm to about 100 nm over the dummy siliconsubstrate 20.

Next, the dummy silicon substrate 20 is unloaded out of the chamber. Atthis time, the surface of the Zr target 10 is not cleaned by sputteringperformed in an Ar gas ambient, for example. Thus, the Zr oxide layer 11remains in the surface of the Zr target 10.

Then, as shown in FIG. 1B, a p-silicon substrate 30 is loaded into thechamber. In this case, an isolation region 31 has been definedbeforehand in the surface of the p-silicon substrate 30 by a knownmethod.

After that, a sputtering process is performed using the Zr target 10,including the Zr oxide layer 11 in its surface region, for about threeseconds with an Ar gas ambient created in the chamber. In this manner, afirst Zr oxide film 32 is deposited to a thickness of about 1 to 2 nmover the p-silicon substrate 30. In this process step, the Zr oxidelayer 11 in the surface of the Zr target 10 is subjected to thesputtering process, thereby depositing the first Zr oxide film 32 overthe substrate 30. Thus, the first Zr oxide film 32 can be depositedwithout performing a reactive sputtering process in an ambientcontaining an O₂ gas. Accordingly, no silicon dioxide film will beformed between the p-silicon substrate 30 and first Zr oxide film 32. Inaddition, since the first Zr oxide film 32 has a thickness of at mostabout 1 to 2 nm, the Zr oxide layer 11 still remains in the surface ofthe Zr target 10 even after the first Zr oxide film 32 has beendeposited.

Subsequently, the gas discharge is continued for the sputtering purposefrom the process step shown in FIG. 1B (which will be herein referred toas the “first Zr oxide deposition step”), while an O₂ gas is introducedinto the chamber with the Ar gas, which has been used for the first Zroxide deposition step, left in the chamber. Then, as shown in FIG. 1C, areactive sputtering process can be performed using the Zr target 10 witha mixed gas ambient containing Ar and O₂ gases created in the chamber.As a result, a second Zr oxide film 33 is deposited to a thickness ofabout 3 to 5 nm over the first Zr oxide film 32. That is to say, thefirst Zr oxide deposition step and the process step shown in FIG. 1C(which will be herein referred to as the “second Zr oxide depositionstep”) can be performed continuously.

In the first embodiment, the second Zr oxide film 33 is deposited tohave the same oxygen concentration as the first Zr oxide film 32 bycontrolling the flow rate of the O₂ gas during the second Zr oxidedeposition step, for example. Specifically, ZrO₂ films with thestoichimetric composition, for example, may be deposited as the firstand second Zr oxide films 32 and 33 so that the oxygen concentrations ofthe first and second Zr oxide films 32 and 33 are equal to each other,i.e., about 66 at %.

Then, the p-silicon substrate 30 is unloaded out of the chamber. At thistime, the surface of the Zr target 10 is not cleaned by a sputteringprocess performed in an Ar gas ambient, for example. Thus, the Zr oxidelayer 11 remains on the surface of the Zr target 10. Accordingly,process steps similar to the first and second Zr oxide deposition stepscan be performed without performing the process step shown in FIG. 1A(which will be herein referred to as a “target oxidation step”). That isto say, after at least one dummy silicon wafer has been subjected to thetarget oxidation step using a sputtering system of a single waferprocessing type, Zr oxide films to be a gate insulating film can bedeposited over multiple silicon wafers numerous times without allowingany silicon dioxide film to exist under the Zr oxide films. Accordingly,it is possible to form the gate insulating film consisting essentiallyof the high-dielectric-constant material alone while minimizing the timefor performing various process steps other than the steps of forming thegate insulating film and the number of dummy wafers. That is to say, thegate insulating film consisting essentially of thehigh-dielectric-constant material alone can be formed at a highthroughput and at a low cost.

After that, as shown in FIG. 1D, a gate electrode 35 is formed over thep-silicon substrate 30 with the gate insulating film 34, made up of thefirst and second Zr oxide films 32 and 33, interposed therebetween.Then, an insulating side-wall 36 is formed on the side faces of the gateelectrode 35. Subsequently, a doped layer 37 to be source/drain regionsis defined in the surface regions of the p-silicon substrate 30.Thereafter, an interlevel dielectric film 38 is deposited over thep-silicon substrate 30 as well as over the gate electrode 35. Then,interconnects 39 including plugs, which are formed in the interleveldielectric film 38 and connected to the doped layer 37, are formed onthe interlevel dielectric film 38, thereby completing an NMOSFET.

FIG. 2 shows variations in flow rates of Ar and O₂ gases with time and avariation in gas-discharge power for sputtering with time for aninterval starting from the first Zr oxide deposition step and endingwith the second Zr oxide deposition step in the method of the firstembodiment.

As shown in FIG. 2, the flow rate of the Ar gas is kept constant fromthe start point of the first Zr oxide deposition step through the endpoint of the second Zr oxide deposition step. On the other hand, the O₂gas starts to be introduced at the end point of the first Zr oxidedeposition step, i.e., the start point of the second Zr oxide depositionstep. Then, after the flow rate of the O₂ gas has reached apredetermined value (i.e., the mixture ratio of the Ar and O₂ gases isfixed), the flow rate of the O₂ gas will be kept constant until thesecond Zr oxide deposition step is over. In this case, the mixture ratioof the Ar and O₂ gases (in the steady state) is not limited to aparticular value. For example, the ratio of the flow rate of the Ar gasto that of the O₂ gas may range from about 7:3 to about 1:9.

As also shown in FIG. 2, the gas-discharge power for sputtering is keptconstant from the start point of the first Zr oxide deposition stepthrough the end point of the second Zr oxide deposition step.

It should be noted that when the second Zr oxide deposition step isover, the Ar and O₂ gases are both exhausted from the chamber and thegas-discharge for sputtering is stopped.

In the first embodiment, the flow rate of the O₂ gas is relatively lowat the initial stage of the second Zr oxide deposition step (see FIG.2). However, since the Zr oxide layer 11 has been formed in the surfaceregion of the Zr target 10, the lower part of the second Zr oxide film33 can also have a predetermined oxygen concentration through thesputtering of the Zr oxide layer 11. Accordingly, the oxygenconcentration of the second Zr oxide film 33 can be uniformized in thethickness direction.

Also, in the first embodiment, oxygen defects might exist in the firstZr oxide film 32 at the start point of the second Zr oxide depositionstep. However, these oxygen defects disappear when exposed to a plasmacreated from the O₂ gas during the second Zr oxide deposition step.Thus, the quality of the first Zr oxide film 32 does not degrade.

FIG. 3 shows the oxygen concentration in the thickness direction of thefirst and second Zr oxide films 32 and 33 formed by the method of thefirst embodiment. In FIG. 3, the thickness is measured from the surfaceof the p-type silicon substrate 30.

As shown in FIG. 3, the first and second Zr oxide films 32 and 33 havethe same oxygen concentration. Specifically, the oxygen concentration ofthe films 32 and 33 is set equal to a predetermined value C (which isuniform in the thickness direction).

As described above, according to the first embodiment, a sputteringprocess is performed using a Zr target 10, including a Zr oxide layer 11in its surface region, with an Ar gas ambient created in a chamber. Inthis manner, a first Zr oxide film 32 to be the lower part of a gateinsulating film 34 is deposited over a p-silicon substrate 30. Afterthat, a reactive sputtering process is performed using the same Zrtarget 10 with a mixed gas ambient containing Ar and O₂ gases created inthe chamber. In this manner, a second Zr oxide film 33 to be the middleor upper part of the gate insulating film 34 is deposited over the firstZr oxide film 32. Accordingly, in the first Zr oxide deposition step,i.e., the initial stage of the process for forming the gate insulatingfilm, no reactive sputtering process is performed but a sputteringprocess is performed using the Zr target 10, including the Zr oxidelayer 11 in its surface region, in an ambient containing no O₂ gas. As aresult, the first Zr oxide film 32 can be deposited over the p-siliconsubstrate 30 without allowing any silicon dioxide film to exist on thesurface of the p-silicon substrate 30. In the second Zr oxide depositionstep, i.e., after the initial stage of the process for forming the gateinsulating film is over, a reactive sputtering process is performed inan ambient containing an O₂ gas with the surface of the p-siliconsubstrate 30 covered with the first Zr oxide film 32. Thus, the secondZr oxide film 33 can be deposited over the first Zr oxide film 32without allowing any silicon dioxide film to exist on the surface of thep-silicon substrate 30. Accordingly, the gate insulating film can beessentially made up of the first and second Zr oxide films 32 and 33alone. In other words, a gate insulating film consisting essentially ofa high-dielectric-constant material alone can be formed. As a result,the resultant MOSFET can have its gate capacitance increased and itsdrivability enhanced. Furthermore, a gate leakage current can beminimized because the gate insulating film can be thick enough with adesired gate capacitance maintained.

Also, according to the first embodiment, before the p-silicon substrate30 is loaded into the chamber, a reactive sputtering process isperformed provisionally using an unoxidized Zr target 10 with a mixedgas ambient containing Ar and O₂ gases created in the chamber. In thismanner, the surface region of the Zr target 10 can be oxidized easily.In this process step, the provisional reactive sputtering is performedon a dummy silicon substrate 20 that has been loaded into the chamberbeforehand. Thus, no insulating Zr oxide is deposited on a wafer stage(which will be used as a gas-discharge electrode during the sputteringprocess) in the chamber when the Zr target 10 is oxidized. As a result,it is possible to avoid the inability to apply a voltage to thep-silicon substrate 30 in subsequent process steps.

In addition, according to the first embodiment, the second Zr oxidedeposition step includes the step of continuing the gas discharge forthe sputtering purpose from the first Zr oxide deposition step, whileintroducing an O₂ gas into the chamber with the Ar gas, used for thefirst Zr oxide deposition step, left in the chamber. Accordingly, thefirst and second Zr oxide deposition steps can be performedcontinuously. As a result, the throughput of the process improves.

In the first embodiment, the Ar gas is used as an inert gas. However,the same effects are attainable even if Xe (xenon) or any other inertgas is used instead of Ar gas.

Also, in the first embodiment, the Zr target 10 is used. Alternatively,any other metal such as Hf, La, Ta or Al, which can produce an oxidewith a high dielectric constant through a reactive sputtering process,may be used for the target because similar effects are achieved with anyof these metals.

Further, in the first embodiment, the Zr oxide layer 11 is formed in thesurface region of the Zr target 10 by performing the reactive sputteringprocess provisionally using the unoxidized Zr target 10 with the mixedgas ambient containing the Ar and O₂ gases created in the chamber.Alternatively, a target of a metal such as Zr, at least the surfaceregion of which has been oxidized, may be prepared in the chamber.

In addition, in the first embodiment, Zr oxide films are supposed tomake up the gate insulating film 34. Alternatively, a Zr oxide filmcontaining Si or any other metal oxide film containing Si may be used asthe gate insulating film 34. Then, the lattice strain created in part ofthe p-silicon substrate 30 in contact with the gate insulating film 34can be relaxed. As a result, decrease in carrier mobility can besuppressed. In this case, Si may be added to a metal oxide film such asZr oxide film to be the gate insulating film by performing a reactivesputtering process using a metal target of, e.g., Zr containing Si.Alternatively, a re-active sputtering process may be performed using atarget of Si as well as a target of a metal such as Zr.

Furthermore, in the first embodiment, the oxygen concentration of thesecond Zr oxide film 33 is controlled by adjusting the flow rate of theO₂ gas in the second Zr oxide deposition step. But the oxygenconcentration of the first Zr oxide film 32 may be controlled byadjusting the flow rate of the O₂ gas during the target oxidation step(the process step shown in FIG. 1A), i.e., by adjusting the oxygenconcentration of the Zr oxide layer 11 formed in the surface region ofthe Zr target 10.

Modified Example of Embodiment 1

Hereinafter, a semiconductor device and a method for fabricating thedevice according to a modified example of the first embodiment of thepresent invention will be described with reference to the drawings.

The method of this modified example is different from the method of thefirst embodiment in the oxygen concentrations of the first and second Zroxide films 32 and 33.

Specifically, in the first embodiment, the second Zr oxide film 33 isdeposited to have the same oxygen concentration as that of the first Zroxide film 32 as shown in FIG. 3, for example.

On the other hand, in the modified example of the first embodiment, thesecond Zr oxide film 33 is deposited to have a different oxygenconcentration from that of the first Zr oxide film 32 by controlling theflow rate of the O₂ gas during the second Zr oxide deposition step, forexample.

FIG. 4 shows the oxygen concentrations in the thickness direction of thefirst and second Zr oxide films 32 and 33 deposited by the method of themodified example of the first embodiment. In FIG. 4, the thickness ismeasured from the surface of the p-type silicon substrate 30.

As shown in FIG. 4, the first Zr oxide film 32 has an oxygenconcentration higher than that of the second Zr oxide film 33.Specifically, the oxygen concentration of the first Zr oxide film 32 isset to a first predetermined value C1 (which is constant in thethickness direction). On the other hand, the oxygen concentration of thesecond Zr oxide film 33 is set to a second predetermined value C2, whichis also constant in the thickness direction. In this case, C1>C2.

According to the modified example of the first embodiment, the followingeffects are attained as well as those obtained by the method of thefirst embodiment.

Specifically, the second Zr oxide film 33 is deposited to have adifferent oxygen concentration from that of the first Zr oxide film 32by controlling the flow rate of the O₂ gas during the second Zr oxidedeposition step. Thus, the structure of the resultant gate insulatingfilm 34, made up of the first and second Zr oxide films 32 and 33, canbe optimized with the reliability and the dielectric constant of thegate insulating film 34 both taken into account. As a result, a highlyreliable, high-performance MOSFET is realized.

In this modified example, the first Zr oxide film 32 has an oxygenconcentration higher than that of the second Zr oxide film 33 as shownin FIG. 4. Alternatively, the oxygen concentration of the first Zr oxidefilm 32 may be lower than that of the second Zr oxide film 33.

Further, in this example, the oxygen concentration of the second Zroxide film 33 is controlled by adjusting the flow rate of the O₂ gasduring the second Zr oxide deposition step. But the oxygen concentrationof the first Zr oxide film 32 may be controlled by adjusting the flowrate of the O₂ gas during the target oxidation step, i.e., by adjustingthe oxygen concentration of the Zr oxide layer 11 formed in the surfaceregion of the Zr target 10.

Embodiment 2

Hereinafter, a semiconductor device and a method for fabricating thedevice according to a second embodiment of the present invention will bedescribed with reference to the drawings.

The method of the second embodiment is different from the method of thefirst embodiment in when the second Zr oxide deposition step (see FIG.1C) is started after the first Zr oxide deposition step (see FIG. 1B) isover.

Specifically, in the first embodiment shown in FIG. 2, even after thefirst Zr oxide deposition step is over, the gas-discharge is continuedfor the sputtering purpose, while the O₂ gas is introduced into thechamber with the Ar gas, which has been used for the first Zr oxidedeposition step, left in the chamber. In this manner, the second Zroxide deposition step, or the process step of depositing the second Zroxide film 33 over the first Zr oxide film 32, is performed. That is tosay, in the first embodiment, the first and second Zr oxide depositionsteps are performed continuously.

On the other hand, in the second embodiment, the gas-discharge forsputtering is suspended during the interval between the first and secondZr oxide deposition steps. In the meantime, the O₂ gas is introducedinto the chamber with the Ar gas, which has been used for the first Zroxide deposition step, left in the chamber. In this case, thegas-discharge is suspended until the flow rate of the O₂ gas introducedinto the chamber reaches, and settles at a predetermined value, e.g.,for about 10 to 15 seconds. When the flow rate of the O₂ gas reaches thepredetermined value, i.e., when the mixture ratio of the Ar and O₂ gasesis fixed, the gas-discharge is resumed, thus starting the second Zroxide deposition step. That is to say, in the second embodiment, a nodischarge interval for changing the ambient in the chamber is placedbetween the first and second Zr oxide deposition steps.

FIG. 5 shows variations in flow rates of the Ar and O₂ gases with timeand a variation in gas-discharge power for sputtering with time for aninterval starting from the first Zr oxide deposition step and endingwith the second Zr oxide deposition step in the method of the secondembodiment.

As shown in FIG. 5, the flow rate of the Ar gas is kept constant fromthe beginning of the first Zr oxide deposition step through the end ofthe second Zr oxide deposition step. On the other hand, the O₂ gasstarts to be introduced when the first Zr oxide deposition step is over.Then, on and after the mixture ratio of the Ar and O₂ gases is fixed atthe start point of the second Zr oxide deposition step, the flow rate ofthe O₂ gas will be kept constant until the second Zr oxide depositionstep is over. In this case, the mixture ratio of the Ar and O₂ gases (inthe steady state) is not limited to any particular value. For example,the ratio of the flow rate of the Ar gas to that of the O₂ gas may rangefrom about 7:3 to about 1:9.

As also shown in FIG. 5, the gas-discharge power for sputtering is keptconstant while each of the first and second Zr oxide deposition steps isbeing performed. But the gas-discharge power is kept off (i.e., reducedto zero) for the interval between the first and second Zr oxidedeposition steps.

It should be noted that when the second Zr oxide deposition step isover, the Ar and O₂ gases are both exhausted from the chamber and thegas-discharge for sputtering is stopped.

According to the second embodiment, the following effects are attainedas well as those obtained by the first embodiment.

Specifically, in the interval between the first and second Zr oxidedeposition steps, the gas-discharge for sputtering is suspended, whilethe O₂ gas is introduced into the chamber with the Ar gas, which hasbeen used for the first Zr oxide deposition step, left in the chamber.Thus, the mixture ratio of the Ar and O₂ gases can be fixed before thesecond Zr oxide deposition step is started. As a result, the oxygenconcentration of the second Zr oxide film 33 is controllable moreeasily.

For the second embodiment, the first and second Zr oxide films 32 and 33may have the same oxygen concentration as in the first embodiment (seeFIG. 3). Alternatively, as in the modified example of the firstembodiment, the first and second Zr oxide films 32 and 33 may havemutually different oxygen concentrations (see FIG. 4).

Embodiment 3

Hereinafter, a semiconductor device and a method for fabricating thedevice according to a third embodiment of the present invention will bedescribed with reference to the drawings.

The method of the third embodiment is different from that of the firstembodiment in the following two respects. Firstly, the second Zr oxidedeposition step (see FIG. 1C) is started at a different time after thefirst Zr oxide deposition step (see FIG. 1B) is over. Secondly, themixture of the Ar and O₂ gases is produced in a different manner for thesecond Zr oxide deposition step.

Specifically, in the first embodiment shown in FIG. 2, even after thefirst Zr oxide deposition step is over, the gas-discharge is continuedfor the sputtering purpose, while the O₂ gas is introduced into thechamber with the Ar gas, which has been used for the first Zr oxidedeposition step, left in the chamber. In this manner, the second Zroxide deposition step, or the process step of depositing the second Zroxide film 33 over the first Zr oxide film 32, is performed. That is tosay, in the first embodiment, the first and second Zr oxide depositionsteps are performed continuously. In addition, the Ar gas that has beenused for the first Zr oxide deposition step is reused as part of themixture for use in the second Zr oxide deposition step.

On the other hand, in the third embodiment, the gas-discharge forsputtering is suspended during the interval between the first and secondZr oxide deposition steps. In the meantime, the Ar gas that has beenused for the first Zr oxide deposition step is exhausted from thechamber, and then an Ar gas is newly introduced along with the O₂ gasinto the chamber. In this case, the gas-discharge is suspended until therespective flow rates of the Ar and O₂ gases introduced into the chamberreach, and are settled at, predetermined values (which may be mutuallydifferent), e.g., for about 10 to 15 seconds. When the flow rates of theAr and O₂ gases reach the predetermined values, i.e., when the mixtureratio of the Ar and O₂ gases is fixed, the gas-discharge is resumed,thus starting the second Zr oxide deposition step. That is to say, inthe third embodiment, a no discharge interval for changing the ambientin the chamber is placed between the first and second Zr oxidedeposition steps. In addition, the Ar gas is newly introduced as part ofthe mixture for use in the second Zr oxide deposition step withoutreusing the Ar gas that has been used for the first Zr oxide depositionstep.

FIG. 6 shows variations in flow rates of the Ar and O₂ gases with timeand a variation in gas-discharge power for sputtering with time for aninterval starting from the first Zr oxide deposition step and endingwith the second Zr oxide deposition step in the method of the thirdembodiment.

As shown in FIG. 6, the flow rate of the Ar gas is kept constant whilethe first Zr oxide deposition step is being performed. However, once thefirst Zr oxide deposition step is over, the Ar gas is exhausted from thechamber so as to have its flow rate reduced to zero before the second Zroxide deposition step is started. On the other hand, the O₂ and new Argases start to be introduced into the chamber after the first Zr oxidedeposition step is over. Then, on and after the mixture ratio of the Arand O₂ gases has been fixed at the start point of the second zr oxidedeposition step, the flow rates of the Ar and O₂ gases are kept constantuntil the second Zr oxide deposition step is over. In this case, themixture ratio of the Ar and O₂ gases (in the steady state) is notlimited to any particular value. For example, the ratio of the flow rateof the Ar gas to that of the O₂ gas may range from about 7:3 to about1:9.

As also shown in FIG. 6, the gas-discharge power for sputtering is keptconstant while each of the first or second Zr oxide deposition steps isbeing performed. But the gas-discharge power is kept off (i.e., reducedto zero) for the interval between the first and second Zr oxidedeposition steps.

It should be noted that when the second Zr oxide deposition step isover, the Ar and O₂ gases are both exhausted from the chamber and thegas-discharge for sputtering is stopped.

According to the third embodiment, the following effects are attained aswell as those obtained by the first embodiment.

Specifically, during the interval between the first and second Zr oxidedeposition steps, the gas-discharge for sputtering is suspended. In themeantime, the Ar gas used for the first Zr oxide deposition step isexhausted from the chamber, and then an Ar gas is newly introduced alongwith the O₂ gas into the chamber. Thus, the mixture ratio of the Ar andO₂ gases should be fixed before the second Zr oxide deposition step isstarted. As a result, the oxygen concentration of the second Zr oxidefilm 33 is controllable much more easily.

For the third embodiment, the first and second Zr oxide films 32 and 33may have the same oxygen concentration as in the first embodiment (seeFIG. 3). Alternatively, as in the modified example of the firstembodiment, the first and second Zr oxide films 32 and 33 may havemutually different oxygen concentrations (see FIG. 4).

Also, in the third embodiment, after the first Zr oxide deposition stepis over, the new Ar gas is introduced along with the O₂ gas into thechamber. In this case, the Ar and O₂ gases may be separately introducedinto the chamber and then mixed together therein. Alternatively, the Arand O₂ gases may be pre-mixed together outside of the chamber and thenintroduced into the chamber. In the latter case, the mixture ratio ofthe Ar and O₂ gases can be fixed in a shorter time. Thus, the nodischarge interval between the first and second Zr oxide depositionsteps can be shortened, thereby improving the throughput of the process.

What is claimed is:
 1. A method for fabricating a semiconductor device,the method comprising the steps of: a) preparing a metal target in achamber, at least a surface region of the target having been oxidized;b) performing a sputtering process using the metal target with an inertgas ambient created in the chamber, thereby depositing a first metaloxide film as a lower part of a gate insulating film over asemiconductor substrate; and c) performing a reactive sputtering processon the metal target with a mixed gas ambient, containing the inert gasand an oxygen gas, created in the chamber, thereby depositing a secondmetal oxide film as a middle or upper part of the gate insulating filmover the first metal oxide film.
 2. The method of claim 1, wherein thestep a) comprises the step of performing a provisional reactivesputtering process on the metal target to be oxidized with a mixed gasambient, containing the inert and oxygen gases, created in the chamber,thereby oxidizing the surface region of the metal target before thesemiconductor substrate is loaded into the chamber.
 3. The method ofclaim 2, wherein the provisional reactive sputtering process isperformed on another semiconductor substrate that has been loaded intothe chamber before the step a) is started.
 4. The method of claim 1,wherein the step c) comprises the step of introducing the oxygen gasinto the chamber with the inert gas, used in the step b), left in thechamber, and with a gas-discharge continued from the step b) to carryout the reactive sputtering process.
 5. The method of claim 1, furthercomprising, between the steps b) and c), the step of introducing theoxygen gas into the chamber with the inert gas, used in the step b),left in the chamber, and with a gas-discharge for the sputtering processsuspended.
 6. The method of claim 1, further comprising, between thesteps b) and c), the step of exhausting the inert gas, used in step b),from the chamber and then newly introducing the inert gas along with theoxygen gas into the chamber with a gas-discharge for the sputteringprocess suspended.
 7. The method of claim 1, wherein the step c)comprises the step of supplying the oxygen gas at a controlled flow rateinto the chamber to deposit the second metal oxide film with a differentoxygen concentration from that of the first